Knowledge of rules governing specificity of synaptic connections among neurons is essential to understanding of ontogeny, regeneration, learning, and some kinds of neuropathology. Previous studies of genesis of neuronal interactions have characterized patterns of synapses between populations of neurons. In the present study I investigate rules for genesis of central and peripheral synapses of individually identified motoneurons of the crayfish abdominal nerve cord. These neurons, like many other neurons in invertebrates, are reproducibly identifiable from animal to animal, and have characteristic neuronal morphology and synaptic connections. The study has three parts: 1) What are the variations is synaptic connections of homologous neurons in the abdomen? Results of this study will suggest mechanisms for generating differential connectivity that are operative during ontogeny. 2) What is the time course of development of these neurons and of their synapses during organogenesis? This project investigates the roles of afferent fibers and of movements of somata in generating each neuron's characteristic geometry. 3) Does each of the motoneurons invariably form all of its characteristic synapses in every animal of the species? This project examines the precision with which the developmental program specifies which synapses will be formed. In sum, these projects explore the usefulness of an invertebrate nervous system for investigating mechanisms for specification of synapses.